1. Field of the Invention
Embodiments of the invention described herein pertain to the field of submersible pump motors. More particularly, but not by way of limitation, one or more embodiments of the invention enable a motor bearing for electric submersible motors.
2. Description of the Related Art
Electric motors convert electrical energy into mechanical energy to produce linear force or torque and are used in many applications requiring mechanical power, such as pumps. In the case of an electric submersible pump (ESP), a multi-phase electric motor is typically used in conjunction with a centrifugal pump to lift fluid, such as oil or water, to the surface of a well. In particular, an ESP motor is typically a two-pole, three-phase, squirrel cage induction motor. The two-pole design conventionally runs at 3600 rpm synchronous speed at 60 Hz power. These electric motors include a stationary component known as a stator, and a rotating component known as the motor shaft. In ESP applications, the stator is energized by a power source located at the well surface and connected to the stator with an electric cable. The electricity flowing through the stator windings generates a magnetic field, and the motor shaft rotates in response to the magnetic field created in the energized stator. A rotor secured to the shaft rotates within the stator. The length of the wound stator determines the number of rotor sections.
Rotor sections are spaced apart from one another, and a stator bearing is located between each rotor section for maintaining the shaft in axial alignment. The bearings are sometimes interchangeably referred to as “motor bearings”, “rotor bearings” or “stator bearings.” These stator bearings are non-rotating bearings that fit snuggly inside the stator bore. The rotating shaft has the rotor sections and bearing sleeves keyed to the shaft. The bearing sleeves rotate inside the stator bearings and prevent the rotors from making contact with the stator bore. The motor is filled with high dielectric oil, and the bearings are hydrodynamic. A pressure wedge in a radial direction is generated between the stator bearing and bearing sleeve while the shaft is rotating and no contact between the stator bearing and the bearing sleeve should occur during proper operation.
It is critical that the stator bearings do not rotate against the stator bore or failure will occur. Should the stator bearings rotate against the stator bore, the roughness of the stator laminations will not provide a sufficient hydrodynamic profile, and this will cause severe wear and lead to failure. However, the bearings must be free to move along the shaft in an axial direction due to thermal expansion. Because the motor components are made of varying materials (the rotors are copper, the shaft is steel), they expand at different rates. If the bearings become locked axially in the bore, the motor will fail due to excessive friction as the rotor tries to expand against a locked bearing.
Two conventional approaches to prevent bearing spin (rotation about the central axis of the bearing) have been used in the ESP industry. One conventional method is to fit an elastomeric band inside a groove cut on the outside of the bearing. The band protrudes above the bearing surface enough to prevent the bearing from spinning through the use of friction, and still allows the bearing to move axially along the motor shaft. However, the elastomeric bands soften with high temperatures, particularly those high temperatures experienced in downhole wells, and the elastomeric bands degrade over time. Degradation of the elastomeric bands causes the bearing to undesirably rotate, eventually leading to failure.
Another approach has been to fit keys on the outside of the bearing that fit inside a continuous keyway in the stator. Some keys are welded in place, while others use spring loaded keys. Once the bearings begin to rotate, the keys will pop into the keyway. The problem with keys is that they have very little surface area and are prone to fail due to fatigue from fretting. If the keys fail, the bearing will rotate against the stator bore and eventually cause a failure. In addition, keys have a tendency to pound an indentation in the keyway. The keys are then locked axially into place, which undesirably prevents the bearing from moving in an axial direction.
As is apparent from the above, current approaches to prevent rotation of motor bearings suffer from many shortcomings. Therefore, there is a need for an improved motor bearing for electric submersible motors.